The present invention relates generally to mechanical assemblies and particularly to mechanical assemblies that achieve rapid and accurate linear translatory motion from a rotary input.
It is well known that one approach to moving an element back and forth along a straight line is to use a lead screw and follower arrangement. Typically such lead screw is formed as a rod having a standard machine thread, and the follower is a nut or tapped element having the same thread as the lead screw. The orientation of the nut is maintained and the lead screw rotated, thereby imparting translational motion to the follower as it is driven by the continuous thread along the rotating rod. In order to provide the necessary strength, the lead screw and follower elements are generally formed of steel. Additionally, to avoid backlash a tight fit is usually present between the nut and threaded rod. Accordingly, a relatively high amount of friction is present. Lubricants can reduce this only a certain amount, and the remaining friction in the mechanism must simply be tolerated. Additionally, in order to permit a useful amount of translational travel, the typical lead screw is quite long in relation to its diameter and, thus, there is a certain bending or deflection present along the unsupported length of the lead screw.
If a high thread angle is used to permit fewer turns of the thread to produce a given linear motion, the friction becomes high.
Moreover, since the lead screw and follower are formed of steel there are relatively large inertial forces present, and the speed of response for the assembly, due to the mass of the lead screw and follower, is relatively slow.